c# - Regex - match a string, but only where the next word is not 'x'

If I have this string:

"The quick brown fox jumped over the lazy dog. What a nice brown fox that is."

What regex would I use to match the text 'brown fox' but not where the following word is 'that', i.e. (matches in italic):

"The quick brown fox jumped over the lazy dog. What a nice brown fox that is."

Answer

You need a zero-width negative lookahead assertion, i.e.,

brown fox(?! that)